1. Field of the Invention
The present invention relates to a cylinder head structure of an engine in which a cooling medium is supplied to a cylinder head.
2. Description of the Related Art
An engine of the type that cooling water discharged from a cooling pump is supplied to a cylinder head before it is supplied to a cylinder block to efficiently cool the cylinder head has been disclosed in Japanese Utility Model Examined Publication No. 4-44816 (refer to the fourth line in the fifth row in the right column of the third page to the third line in the ninth row of the right column of the fifth page, and FIGS. 1 to 6). In such an engine, cooling water is supplied from a cooling water inlet provided at an end of the cylinder head to a cooling water passage in the cylinder head by the cooling water pump. The cooling water which has flown through the cooling water passage in the cylinder head is caused to pass through the cylinder block via a cooling water outlet passage on the cylinder head side, and pass through a cooling water passage in the cylinder block, and is then retuned to a radiator.
To cool the cylinder head to improve an octane value, and on the other hand, to prevent the viscosity of lubricant flowing in the cylinder block from being excessively increased due to excessive cooling of the cylinder block, a cooling water flow control valve controls the volume of cooling water flowing through the cooling water passage in the cylinder block so that the temperature of the cylinder block can be kept at an appropriate temperature.
The cooling water supplied via one end of the cylinder head swiftly flows in the direction in which cylinders are arranged, and is divided into a flow toward intake ports and a flow toward exhaust ports by spark plug mounting parts disposed at the centers of the respective cylinders. In the cylinder head, however, the temperature of the spark plug mounting parts and the temperature of the exhaust ports are high, and hence the cooling water heats up while flowing along the spark plug mounting parts, and the intake ports and the exhaust ports cannot be efficiently cooled.
The temperature of the intake ports affects the density of gas supplied into the cylinders. Specifically, if the intake ports cannot be sufficiently cooled, the density of gas supplied into cylinders lowers and decreases engine output.